1. Field of the Invention
The present invention relates to an optical fiber and a method of measuring polarization mode dispersion of an optical fiber.
2. Description of Related Art
In view of recent progress with respect to increasing transmission speed, and lengthening of transmission distance in optical communications, reduction of polarization mode dispersion (hereinafter, abbreviated to as “PMD”) is advantageous for optical fibers, that form transmission paths.
The PMD of the optical fiber is mode dispersion caused a difference in group velocity between two orthogonal intrinsic polarization components, which propagate in the optical fiber. One source of mode dispersion is imperfections of the fiber optic, such as non-circularity of the core shape of the optical fiber, asymmetry of stress occurring in the core, etc.
There are two parameters for determining the PMD. One parameter is the magnitude of local birefringence of the optical fiber, and the other is polarization mode coupling which expresses how the direction of a birefringent axis of the optical fiber changes in the longitudinal direction of the optical fiber.
The magnitude of local birefringence of the optical fiber can be quantified by using a beat length, LB. Beat length indicates a propagation distance required for polarization in the optical fiber to rotate 360 degrees and return to its initial polarization state.
Another parameter expressing the local birefringence of the optical fiber is a mode birefringence rate, B. The following Equation (1) defines the relationship between the mode birefringence rate, B, and beat length, LB.
                              L          B                =                  λ          B                                    (        1        )            
In Equation (1), above, λ equals the wavelength of light.
When the length of the optical fiber is short, polarization mode coupling can be considered negligible. Therefore, for a short fiber optic cable, the PMD reduces to the expression according to the following Equation (2). As shown below, PMD is a function of light velocity, C, and the length, L, of the optical fiber.
                    PMD        =                              λ                                          L                B                            ·              C                                ·          L                                    (        2        )            
For a short fiber optic cable, Equation (2) provides that the PMD increases in proportion to the length, L, of the optical fiber.
On the other hand, when the length of the optical fiber is long, the PMD is expressed by the following Equation (3).
                    PMD        =                              λ                                          L                B                            ·              C                                ⁢                                    L              ·                              L                C                                                                        (        3        )            
In Equation (3), above, LC is an average coupling length, which is a parameter expressing the magnitude of the polarization mode coupling. Equation (3), provides that the PMD increases in proportion to the square root of the length L of the optical fiber. As the polarization mode coupling increases, the magnitude of the average coupling length decreases. The magnitude of the polarization mode coupling is mainly determined by the distortion of the optical fiber, such as, a force applied from the outside.
When the length L of the optical fiber is shorter than LC, the PMD can be expressed by using Equation (2). On the other hand, when the length L of the optical fiber is longer than LC, the PMD can be expressed by using Equation (3).
It can be seen from Equations (2) and (3) that the PMD increases as beat length, LB, becomes shorter, and average coupling length, LC, becomes longer.
The optical fiber is typically transferred to an optical cabling process in a wound around a bobbin state, or is typically shipped and transported as an optical fiber unit. Therefore, it is desirable to be able to measure the PMD in the optical fiber that is in the wound around the bobbin state.
However, by winding the optical fiber around the bobbin, disturbances such as bending, lateral pressure, and distortion, occur in the optical fiber, thereby changing LB and LC, and hence the PMD varies. Therefore, in the same optical fiber, the PMD of the optical fiber wound around the bobbin for transport, and the PMD of the optical fiber after the optical cabling process, show quite different values (for example, see Scott Grindstaff, Joseph Hill, Omid Daneshvar, “Extrinsic Stress Effects on Polarization Mode Dispersion in Optical Fiber Cables”, International Wire & Cable Symposium Proceedings, 1993, pp. 647 to 654). The PMD of the optical fiber after optical cabling increases, and in some cases, exceeds an upper limit of the PMD specified by the standard, thereby causing problems.
Moreover, the optical fiber is shipped to the optical cabling process in a length of about 20 to 100 km, and at the time of optical cabling, the length of the optical fiber is cut to 1 to 10 km. Therefore, if there is a place having locally large PMD, even if the PMD of the whole length of the optical fiber before optical cabling is small, when the optical fiber is cut and formed as a cable, there may be a place where there is large PMD, which can be problematic.